1. Field of the Invention
This invention relates to a process for reforming, where ethylbenzene formed during the reforming is converted to xylenes.
2. Description of the Prior Art
Catalytic reforming is one well-known method of producing C6 to C8 aromatic compounds and involves contacting an aliphatic and/or naphthenic hydrocarbon mixture, such as C5-205° C. naphtha cut from a crude oil distillation unit, with a reforming catalyst. Reforming involves a complex series of chemical reactions, including cracking, dehydrocyclization, dehydrogenation, and isomerization, to produce a product mixture containing a wide variety of aromatic compounds, including benzene, toluene, a C8 aromatic fraction (para-xylene, ortho-xylene, meta-xylenes, and ethylbenzene) and heavy aromatics, such as mesitylene, pseudocumene, ethyltoluenes and other C9–C12 aromatics.
The C8 aromatic fraction of the reformate can vary quite widely in composition, but will usually contain 10 to 32 wt. percent ethylbenzene in the C8 aromatics and a near equilibrium amount of xylenes. The amount of ethylbenzene formed during reforming will depend on the composition of the naphtha. Of the xylene isomers, para-xylene is of particular value as a large volume chemical intermediate in a number of applications, such as the manufacture of terephthalic acid, which is an intermediate in the manufacture of polyester.
The individual xylene isomers of the reformate may be separated by appropriate physical methods. Ortho-xylene may be separated by fractional distillation. Para-xylene is usually recovered in high purity from the C8 aromatic fraction by separating the para-xylene from the ortho-xylene, meta-xylene, and ethylbenzene using separation techniques such as fractional crystallization or selective adsorption, e.g., Parex™ process. The meta-xylene and ortho-xylene remaining after the para-xylene separation are isomerized to produce an equilibrium mixture of xylenes. The para-xylene in the mixture is then separated from the meta-xylene and ortho-xylene and the para-xylene depleted-stream is recycled to extinction to the isomerization unit and then to the para-xylene recovery unit until all of the meta-xylene and ortho-xylene are converted and recovered.
Separation or removal of ethylbenzene from xylene streams is frequently difficult and expensive. One technique for ethylbenzene removal involves the dealkylation of the ethylbenzene to benzene and ethylene. The ethylene produced is saturated to ethane using hydrogen in the presence of a hydrogenation catalyst, such as platinum. Another technique for ethylbenzene reduction involves disproportionation to benzene and diethylbenzene. A disadvantage of these types of conversion is that they result in the formation of benzene. With the current and future anticipated environmental regulations involving benzene, it is usually desirable that conversion not result in the formation of significant quantities of benzene. Another disadvantage of these types of conversion is that they convert eight carbon aromatics, i.e., ethylbenzene, to six carbon aromatics, i.e., benzene, which is less valuable than para-xylene, together with a low value ethane. Still another technique for removing ethylbenzene involves converting ethylbenzene to xylenes. An advantage of this type of conversion is that it results in the formation of higher value product without the formation of benzene.
Frequently, ethylbenzene conversion is carried out during the isomerization of the para-xylene depleted feedstream. Examples of xylenes isomerization/ethylbenzene conversion processes are disclosed in U.S. Pat. Nos. 4,899,011 and 5,082,984.
Ethylbenzene can be formed during reforming from C8 naphthenes, C8 isoalkane and/or C8 isoalkene precursors of ethylbenzene. Examples of such precursors include ethyl-cyclohexane, ethyl-cyclohexenes, 3-ethylhexane, 3-ethylhexenes, 3-ethylhexadienes, 3-ethylhexatriene, 3-methylheptane, 3-methylheptenes, 3-methylheptadienes, 3-methylheptatrienes, octane, octenes, octadienes, octatrienes and/or octatetraenes. The concentration of these precursors in the feed fed to the reformer will affect the amount of ethylbenzene formed during reforming. Generally, the feed will contain from about 1 to about 10 weight percent of ethylbenzene precursors.
The concentration of ethylbenzene in the reformate can affect the efficiency of subsequent xylenes processing operations e.g., para-xylene separation and xylenes isomerization. For example, the processing of a C8 aromatics feed containing 20 weight percent ethylbenzene can result in the total recycle stream to the para-xylene separation unit being increased by about 20 percent over a C8 aromatics feed containing no ethylbenzene. The same increase in recycle stream applies to the xylenes isomerization unit. Thus, the use of a C8 aromatics feed having no or minimal amounts of ethylbenzene in place of one having 20 percent, can debottleneck subsequent xylenes processing units, e.g., xylenes separation units and isomerization units, by about 20 percent.
In modern isomerization units where the ethylbenzene is dealkylated to benzene in the isomerization unit, the ethylbenzene dealkylation reaction usually proceeds at 50 to 85 percent ethylbenzene conversion per pass. Thus, the recycle feed stream provided to the xylenes separation unit always contains a substantial amount of ethylbenzene. This ethylbenzene builds up in the recycled feed stream causing processing equipment to be larger than necessary to merely process the xylenes. Thus, it is usually desirable that the feed to the isomerization unit have reduced amounts of ethylbenzene. The ethylbenzene contained within the recycle stream can be reduced by operating at high ethylbenzene conversions, but high ethylbenzene conversion is usually accompanied by high conversion of xylenes to less desirable toluene and C9+ aromatics. Commercially, an economic optimum ethylbenzene conversion is most often targeted that balances recycle rate and byproduct formation.
The present invention provides a process of reforming a hydrocarbon feed that results in the formation of reformate with reduced amounts of ethylbenzene and increased amounts of xylenes. By achieving reduced ethylbenzene in the xylenes recovery loop, low byproduct yield can be achieved without substantial buildup of ethylbenzene in the xylenes recovery loop and a commensurate reduction of para-xylene production capacity.